Make sure you have Yojimbo.
Make sure you have plenty of money.
During the battle between the dark valefor, make Yuna go first( by switching her, or having her speedhigher than the rest of your party ) summon Yojimbo, now the best thing to do is have his overdrive full up, then pay him something like: 1,500,000 gil, you will then do an ecredible amount of damage which should kill it in one shot.Try this for all the Aeons.
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